That was supposed to be a friendlink which allows to read it in any case. But hackernews has obviously trimmed it. This is the right link: https://medium.com/better-programming/numpy-illustrated-the-...

Their payment model is attractive by its fairness:

A micropayment an author gets is proportional to the amount of time a paid reader spent on reading the article as compared to the total time he spent on medium.

And friends links open free access to those who isn't aware or don't like this model.